Internal lamin structures within G1 nuclei of human dermal fibroblasts

Nucleoplasmic lamins and their interaction partners, LAP2alpha, Rb, and BAF, in transcriptional regulation

Lamins are major structural components of the nuclear envelope in multicellular eukaryotes. Particularly A-type lamins are also located in the nucleoplasm, likely involving a specific binding partner, lamina-associated polypeptide 2alpha (LAP2alpha). LAP2alpha-lamins A/C complexes in the nucleoplasm have been implicated in the regulation of gene expression by various means. They bind chromatin proteins and chromatin modifying enzymes, and can thus participate in epigenetic control pathways. Furthermore, binding of lamins A/C complexes to specific transcription factors and repressors may directly affect their transcriptional activity. LAP2alpha-lamins A/C also regulate retinoblastoma protein and influence cell cycle progression and differentiation, which could have important implications for molecular mechanisms of laminopathic diseases, linked to lamins A/C mutations.

Primary laminopathy fibroblasts display altered genome organization and apoptosis

A number of diseases associated with specific tissue degeneration and premature aging have mutations in the nuclear envelope proteins A-type lamins or emerin. Those diseases with A-type lamin mutation are inclusively termed laminopathies. Due to various hypothetical roles of nuclear envelope proteins in genome function we investigated whether alterations to normal genomic behaviour are apparent in cells with mutations in A-type lamins and emerin. Even though the distributions of these proteins in proliferating laminopathy fibroblasts appear normal, there is abnormal nuclear positioning of both chromosome 18 and 13 territories, from the nuclear periphery to the interior. This genomic organization mimics that found in normal nonproliferating quiescent or senescent cells. This finding is supported by distributions of modified pRb in the laminopathy cells. All laminopathy cell lines tested and an X-linked Emery-Dreifuss muscular dystrophy cell line also demonstrate increased incidences of apoptosis. The most extreme cases of apoptosis occur in cells derived from diseases with mutations in the tail region of the LMNA gene, such as Dunningan-type familial partial lipodystrophy and mandibuloacral dysplasia, and this correlates with a significant level of micronucleation in these cells.

Differential expression of nuclear lamin, the major component of nuclear lamina, during neurogenesis in two germinal regions of adult rat brain

Lamins are major structural proteins of the nuclear envelope. Three lamin subtypes, A/C, B1 and B2, predominate in mammalian somatic cells. While the expression levels of lamins in several tissues are known to change during cell differentiation, lamin expression is poorly understood in the nervous system. To investigate the expression of lamins during neuronal differentiation in the mammalian adult brain, we performed immunohistochemical studies on lamins A/C, B1 and B2 in two neurogenic regions of rat brain: the subgranular zone of the dentate gyrus and the subventricular zone of the lateral ventricle. In particular, three types of cells were analysed using confocal microscopy: GFAP-positive cells as primary progenitor (stem) cells, PSA-NCAM-positive cells as subsequent neuronal progenitor cells, and NeuN-positive mature neurons. GFAP-positive cells possesed lamin A/C (++), B1 (++) and B2 (++), PSA-NCAM-positive cells possessed lamin A/C (-), B1 (+++) and B2 (+), and mature neurons possessed lamin A/C (++), B1 (+) and B2 (+++), in both neurogenic regions. These observations showed that the compositions of expressing lamin subtypes are distinct in particular differentiation stages during neurogenesis in the adult rat brain. Our results suggest that the alteration of nuclear lamina structure is coupled with the progression of neuronal differentiation.

LAP2alpha-binding protein LINT-25 is a novel chromatin-associated protein involved in cell cycle exit

Lamina-associated polypeptide 2alpha (LAP2alpha) is a nuclear protein dynamically associating with chromatin during the cell cycle. In addition, LAP2alpha interacts with A-type lamins and retinoblastoma protein and regulates cell cycle progression via the E2F-Rb pathway. Using yeast two-hybrid analysis and three independent in vitro binding assays we identified a new LAP2alpha interaction partner of hitherto unknown functions, which we termed LINT-25. LINT-25 protein levels were upregulated during G1 phase in proliferating cells and upon cell cycle exit in quiescence, senescence and differentiation. Upon cell cycle exit LINT-25 accumulated in heterochromatin foci, and LAP2alpha protein levels were downregulated by proteasomal degradation. Although LAP2alpha was not required for the upregulation and reorganization of LINT-25 during cell cycle exit, transient expression of LINT-25 in proliferating cells caused loss of LAP2alpha and subsequent cell death. Our data show a role of LINT-25 and LAP2alpha during cell cycle exit, in which LINT-25 acts upstream of LAP2alpha.

The nuclear lamina

The inner face of the nuclear envelope of metazoan cells is covered by a thin lamina consisting of a one-layered network of intermediate filaments interconnecting with a complex set of transmembrane proteins and chromatin associating factors. The constituent proteins, the lamins, have recently gained tremendous recognition, because mutations in the lamin A gene, LMNA, are the cause of a complex group of at least 10 different diseases in human, including the Hutchinson-Gilford progeria syndrome. The analysis of these disease entities has made it clear that besides cytoskeletal functions, the lamina has an important role in the "behaviour" of the genome and is, probably as a consequence of this function, intimately involved in cell fate decisions. Furthermore, these functions are related to the involvement of lamins in organizing the position and functional state of interphase chromosomes as well as to the occurrence of lamins and lamina-associated proteins within the nucleoplasm. However, the structural features of these lamins and the nature of the factors that assist them in genome organization present an exciting challenge to modern biochemistry and cell biology.

Nuclear envelope defects in muscular dystrophy

Muscular dystrophies are a heterogeneous group of disorders linked to defects in 20-30 different genes. Mutations in the genes encoding a pair of nuclear envelope proteins, emerin and lamin A/C, have been shown to cause the X-linked and autosomal forms respectively of Emery-Dreifuss muscular dystrophy. A third form of muscular dystrophy, limb girdle muscular dystrophy 1b, has also been linked to mutations in the lamin A/C gene. Given that these two genes are ubiquitously expressed, a major goal is to determine how they can be associated with tissue specific diseases. Recent results suggest that lamin A/C and emerin contribute to the maintenance of nuclear envelope structure and at the same time may modulate the expression patterns of certain mechanosensitive and stress induced genes. Both emerin and lamin A/C may play an important role in the response of cells to mechanical stress and in this way may help to maintain muscle cell integrity.

Lamins A and C are present in the nuclei of early porcine embryos, with lamin A being distributed in large intranuclear foci

Gametogenesis and embryogenesis are dynamic developmental stages marked by extensive modifications in the organization of the genome and nuclear architecture. In the literature it is conveyed that only B-type lamins are required in these early stages of development and that A-type lamins are not present or required until differentiation of specific cell types associated with specialized tissue is initiated. To assess the presence of nuclear structures that are putatively involved in genome regulation, we investigated the distribution of lamin proteins throughout the early stages of porcine embryonic development, using testes tissue sections, oocytes and in-vitro fertilized (IVF) porcine embryos and employing anti-lamin antibodies. We have shown that anti-lamin A staining is present at the one-cell, two-cell, four-cell, and six- to eight-cell stages of early porcine embryo development, but diminishes at the morulae and blastocyst stages. Large intranuclear anti-lamin A foci are prominent in the early preimplantation stages. Both anti-lamin A/C and anti-lamin B staining were clearly present in all embryonic stages. Immature porcine oocytes revealed lamin rings using the monoclonal anti-lamin A/C antibody and many immature oocytes exhibited a pale rim staining pattern with anti-lamin A antibody. A-type lamins were not observed in sperm precursor cells. Thus, we have shown that A-type lamins and B-type lamins are present at the nuclear envelope in very early porcine embryos and that lamin A is also found in large intranuclear aggregates in two-cell to eight-cell embryos but is lacking from later embryonic stages.

Nuclear architecture in the light of gene expression and cell differentiation studies

It is evident that primary DNA sequences, that define genomes, are responsible for genome functions. However, the functional properties of chromatin are additionally regulated by heritable modifications known as epigenetic factors and, therefore, genomes should be also considered with respect to their 'epigenomes'. Nucleosome remodelling, DNA methylation and histone modifications are the most prominent epigenetic changes that play fundamental roles in the chromatin-mediated control of gene expression. Another important nuclear feature with functional relevance is the organization of mammalian chromatin into distinct chromosome territories which are surrounded by the interchromatin compartment that is necessary for transport of regulatory molecules to the targeted DNA. The inner structure of the chromosome territories, as well as the arrangement of the chromosomes within the interphase nuclei, has been found to be non-randomly organized. Therefore, a specific nuclear arrangement can be observed in many cellular processes, such as differentiation and tumour cell transformation.

Quantitative analysis of localization and nuclear aggregate formation induced by GFP-lamin A mutant proteins in living HeLa cells

Although A-type lamins are ubiquitously expressed, their role in the tissue-specificity of human laminopathies remains enigmatic. In this study, we generate a series of transfection constructs encoding missense lamin A mutant proteins fused to green fluorescent protein and investigate their subnuclear localization using quantitative live cell imaging. The mutant constructs used included the laminopathy-inducing lamin A rod domain mutants N195K, E358K, M371K, R386K, the tail domain mutants G465D, R482L, and R527P, and the Hutchinson-Gilford progeria syndrome-causing deletion mutant, progerin (LaA delta50). All mutant derivatives induced nuclear aggregates, except for progerin, which caused a more lobulated phenotype of the nucleus. Quantitative analysis revealed that the frequency of nuclear aggregate formation was significantly higher (two to four times) for the mutants compared to the wild type, although the level of lamin fusion proteins within nuclear aggregates was not. The distribution of endogenous A-type lamins was altered by overexpression of the lamin A mutants, coexpression experiments revealing that aberrant localization of the N195K and R386K mutants had no effect on the subnuclear distribution of histones H2A or H2B, or on nuclear accumulation of H2A overexpressed as a DsRed2 fusion protein. The GFP-lamin fusion protein-expressing constructs will have important applications in the future, enabling live cell imaging of nuclear processes involving lamins and how this may relate to the pathogenesis of laminopathies.

Stabilization of the retinoblastoma protein by A-type nuclear lamins is required for INK4A-mediated cell cycle arrest

Mutations in the LMNA gene, which encodes all A-type lamins, including lamin A and lamin C, cause a variety of tissue-specific degenerative diseases termed laminopathies. Little is known about the pathogenesis of these disorders. Previous studies have indicated that A-type lamins interact with the retinoblastoma protein (pRB). Here we probe the functional consequences of this association and further examine links between nuclear structure and cell cycle control. Since pRB is required for cell cycle arrest by p16(ink4a), we tested the responsiveness of multiple lamin A/C-depleted cell lines to overexpression of this CDK inhibitor and tumor suppressor. We find that the loss of A-type lamin expression results in marked destabilization of pRB. This reduction in pRB renders cells resistant to p16(ink4a)-mediated G(1) arrest. Reintroduction of lamin A, lamin C, or pRB restores p16(ink4a)-responsiveness to Lmna(-/-) cells. An array of lamin A mutants, representing a variety of pathologies as well as lamin A processing mutants, was introduced into Lmna(-/-) cells. Of these, a mutant associated with mandibuloacral dysplasia (MAD R527H), as well as two lamin A processing mutants, but not other disease-associated mutants, failed to restore p16(ink4a) responsiveness. Although our findings do not rule out links between altered pRB function and laminopathies, they fail to support such an assertion. These findings do link lamin A/C to the functional activation of a critical tumor suppressor pathway and further the possibility that somatic mutations in LMNA contribute to tumor progression.

Nuclear Lamins: Laminopathies and Their Role in Premature Ageing

It has been demonstrated that nuclear lamins are important proteins in maintaining cellular as well as nuclear integrity, and in maintaining chromatin organization in the nucleus. Moreover, there is growing evidence that lamins play a prominent role in transcriptional control. The family of laminopathies is a fast-growing group of diseases caused by abnormalities in the structure or processing of the lamin A/C ( LMNA) gene. Mutations or incorrect processing cause more than a dozen different inherited diseases, ranging from striated muscular diseases, via fat- and peripheral nerve cell diseases, to progeria. This broad spectrum of diseases can only be explained if the responsible A-type lamin proteins perform multiple functions in normal cells. This review gives an overview of current knowledge on lamin structure and function and all known diseases associated with LMNA abnormalities. Based on the knowledge of the different functions of A-type lamins and associated proteins, explanations for the observed phenotypes are postulated. It is concluded that lamins seem to be key players in, among others, controlling the process of cellular ageing, since disturbance in lamin protein structure gives rise to several forms of premature ageing.

Phosphorylation of the p34(cdc2) target site on goldfish germinal vesicle lamin B3 before oocyte maturation

The nuclear membranes surrounding fish and frog oocyte germinal vesicles (GVs) are supported by the lamina, an internal, mesh-like structure that consists of the protein lamin B3. The mechanisms by which lamin B3 is transported into GVs and is assembled to form the nuclear lamina are not well understood. In this study, we developed a heterogeneous microinjection system in which wild-type or mutated goldfish GV lamin B3 (GFLB3) was expressed in Escherichia coli, biotinylated, and microinjected into Xenopus oocytes. The localization of the biotinylated GFLB3 was visualized by fluorescence confocal microscopy. The results of these experiments indicated that the N-terminal domain plays important roles in both nuclear transport and assembly of lamin B3 to form the nuclear lamina. The N-terminal domain includes a major consensus phosphoacceptor site for the p34(cdc2) kinase at amino acid residue Ser-28. To investigate nuclear lamin phosphorylation, we generated a monoclonal antibody (C7B8D) against Ser-28-phosphorylated GFLB3. Two-dimensional (2-D) electrophoresis of GV protein revealed two major spots of lamin B3 with different isoelectric points (5.9 and 6.1). The C7B8D antibody recognized the pI-5.9 spot but not the pI-6.1 spot. The former spot disappeared when the native lamina was incubated with lambda phage protein phosphatase (lambda-PP), indicating that a portion of the lamin protein was already phosphorylated in the goldfish GV-stage oocytes. GFLB3 that had been microinjected into Xenopus oocytes was also phosphorylated in Xenopus GV lamina, as judged by Western blotting with C7B8D. Thus, lamin phosphorylation appears to occur prior to oocyte maturation in vivo in both these species. Taken together, our results suggest that the balance between phosphorylation by interphase lamin kinases and dephosphorylation by phosphatases regulates the conformational changes in the lamin B3 N-terminal head domain that in turn regulates the continual in vivo rearrangement and remodeling of the oocyte lamina.

Lamin A/C and cardiac diseases

PURPOSE OF REVIEW

In this review, we will outline the most recent and significant findings on the role of the lamin A/C in cardiac diseases.

RECENT FINDINGS

Mutations in the lamin A/C gene (LMNA) are associated with numerous diseases involving the heart, skeletal muscles, bones, adipose and nervous tissues. LMNA is one of the most prevalent genes in dilated cardiomyopathy in which it is associated with a high risk of dysrhythmias, sudden death and heart failure. Lamins A and C interact with several proteins reflecting their multiple functions, some of which are likely still unknown. No abnormalities specific to dilated cardiomyopathy are emerging from investigations of striated muscles biopsies or fibroblasts from LMNA mutation carriers. An early diagnosis of the disease is difficult. Both animal and cellular models tend to confirm that lamins A and C play a key role in maintaining the nuclear architecture as well as in regulating transcription.

SUMMARY

The cardiac phenotype associated to LMNA mutations is now much clearer, but the molecular mechanisms underlying cellular and tissue specific phenotypes are still puzzling. Systematic mutation screenings and cardioverter-defibrillator implantation have been recommended in patients with cardiac symptoms.

Altered splicing in prelamin A-associated premature aging phenotypes

Hutchinson-Gilford progeria (HGPS), a rare and severe developmental disorder characterized by features recalling premature aging, and restrictive dermopathy (RD), a neonatal lethal genodermatosis, have recently been identified as being primary or secondary "laminopathies." These are heterogeneous disorders due to altered function of lamins A/C or related proteins. In physiological conditions, mature lamin A is obtained through a series of post-translational processing steps performed on a protein precursor, prelamin A. The major pathophysiological mechanism involved in progeria is an aberrant splicing of pre-mRNAs issued from the LMNA gene, due to a de novo heterozygous point mutation, leading to the production and accumulation of truncated lamin A precursors. Aberrant splicing of prelamin A pre-mRNAs causing the production of more extensively truncated precursors is involved in the allelic disease restrictive dermopathy. Other restrictive dermopathy cases are due to the inactivation of a key enzyme involved in the maturation of lamin A precursors (ZMPSTE24). In functional terms, all these conditions share the same pathophysiological basis: intranuclear accumulation of lamin A precursors, which cannot be fully processed (due to primary or secondary events) and exert toxic, dominant negative effects on nuclear homeostasis. Most other laminopathies are due to autosomal dominant LMNA point mutations inferred to cause single amino acid substitutions. In any case, the impact of these mutations on pre-mRNA splicing has rarely been assessed. These disorders affect different tissues and organs, mainly including bone, skin, striated muscles, adipose tissue, vessels, and peripheral nerves in isolated or combined fashions, giving rise to syndromes whose severity ranges from mild to perinatally lethal. In this chapter we review the structure and functions of lamins A/C in physiological and pathological conditions, describe their known or putative roles, namely, in the pathogenesis of HGPS and RD in relation to existing animal models, and envisage possible targeted therapeutic strategies on the basis of recent research results.

Lamin A/C expression is a marker of mouse and human embryonic stem cell differentiation

Nuclear lamins comprise the nuclear lamina, a scaffold-like structure that lines the inner nuclear membrane. B-type lamins are present in almost all cell types, but A-type lamins are expressed predominantly in differentiated cells, suggesting a role in maintenance of the differentiated state. Previous studies have shown that lamin A/C is not expressed during mouse development before day 9, nor in undifferentiated mouse embryonic carcinoma cells. To further investigate the role of lamins in cell phenotype maintenance and differentiation, we examined lamin expression in undifferentiated mouse and human embryonic stem (ES) cells. Wide-field and confocal immunofluorescence microscopy and semiquantitative reverse transcription-polymerase chain reaction analysis revealed that undifferentiated mouse and human ES cells express lamins B1 and B2 but not lamin A/C. Mouse ES cells display high levels of lamins B1 and B2 localized both at the nuclear periphery and throughout the nucleoplasm, but in human ES cells, B1 and B2 expression is dimmer and localized primarily at the nuclear periphery. Lamin A/C expression is activated during human ES cell differentiation before downregulation of the pluripotency marker Oct-3/4 but not before the downregulation of the pluripotency markers Tra-1-60, Tra-1-81, and SSEA-4. Our results identify the absence of A-type lamin expression as a novel marker for undifferentiated ES cells and further support a role for nuclear lamins in cell maintenance and differentiation.

Ca2+-dependent nuclear contraction in the heliozoon Actinophrys sol

Ca2+-dependent contractility was found to exist in the nucleus of the heliozoon protozoan Actinophrys sol. Upon addition of Ca2+ ([Ca2+]free = 2.0 x 10(-3) M), diameters of isolated and detergent-extracted nuclei became reduced from 16.5+/-1.7 microm to 11.0+/-1.3 microm. The threshold level of [Ca2+]free for the nuclear contraction was 2.9 x 10(-7) M. The nuclear contraction was not induced by Mg2+, and was not inhibited by colchicine or cytochalasin B. Contracted nuclei became expanded when Ca2+ was removed by EGTA; thus cycles of contraction and expansion could be repeated many times by alternating addition of Ca2+ and EGTA. The Ca2+-dependent nuclear contractility remained even after high salt treatment, suggesting a possible involvement of nucleoskeletal components in the nuclear contraction. Electron microscopy showed that, in the relaxed state, filamentous structures were observed to spread in the nucleus to form a network. After addition of Ca2+, they became aggregated and constructed a mass of thicker filaments, followed by re-distribution of the filaments spread around inside of the nucleus when Ca2+ was removed. These results suggest that the nuclear contraction is induced by Ca2+-dependent transformation of the filamentous structures in the nucleus.

Tight skin and limited joint movements as early presentation of Hutchinson-Gilford progeria in a 7-week-old infant

We present a 7-week-old male infant with pseudoscleroderma as a primary manifestation of the Hutchinson-Gilford syndrome of premature aging. He had suffered intra-uterine growth retardation; micrognathism and a cleft palate were evident at birth. He presented with feeding difficulties and severe, diffuse scleroderma-like lesions, a faint peri-oral cyanosis and prominent scalp veins. With time, special facial features became more and more apparent: frontal bossing, prominent eyes, thin and fine nose and lips, microstomia, low-set ears and occipito-parietal alopecia. Histopathology of the skin showed an increased density and thickness of collagen in the dermis and hypodermis. Within the 1st year of life, typical skeletal characteristics were observed. The diagnosis of Hutchinson-Gilford syndrome was confirmed by analysis of the lamin A gene, revealing a heterozygous c.1824C > T (G608G) mutation.

CONCLUSION

Hutchinson-Gilford syndrome is an extremely rare disorder of which the full clinical spectrum becomes evident with time. Sclerodermatous changes in the infant can be the first manifestation.

Loss of ZMPSTE24 (FACE-1) causes autosomal recessive restrictive dermopathy and accumulation of Lamin A precursors

Restrictive dermopathy (RD) is characterized by intrauterine growth retardation, tight and rigid skin with prominent superficial vessels, bone mineralization defects, dysplastic clavicles, arthrogryposis and early neonatal death. In two patients affected with RD, we recently reported two different heterozygous splicing mutations in the LMNA gene, leading to the production and accumulation of truncated Prelamin A. In other patients, a single nucleotide insertion was identified in ZMPSTE24. This variation is located in a homopolymeric repeat of thymines and introduces a premature termination codon. ZMPSTE24 encodes an endoprotease essential for the post-translational cleavage of the Lamin A precursor and the production of mature Lamin A. However, the autosomal recessive inheritance of RD suggested that a further molecular defect was present either in the second ZMPSTE24 allele or in another gene involved in Lamin A processing. Here, we report new findings in RD linked to ZMPSTE24 mutations. Ten RD patients were analyzed including seven from a previous series and three novel patients. All were found to be either homozygous or compound heterozygous for ZMPSTE24 mutations. We report three novel 'null' mutations as well as the recurrent thymine insertion. In all cases, we find a complete absence of both ZMPSTE24 and mature Lamin A associated with Prelamin A accumulation. Thus, RD is either a primary or a secondary laminopathy, caused by dominant de novo LMNA mutations or, more frequently, recessive null ZMPSTE24 mutations, most of which lie in a mutation hotspot within exon 9. The accumulation of truncated or normal length Prelamin A is, therefore, a shared pathophysiological feature in recessive and dominant RD. These findings have an important impact on our knowledge of the pathophysiology in Progeria and related disorders and will help direct the development of therapeutic approaches.

A Novel Localization of the G-Protein-Coupled CysLT1 Receptor in the Nucleus of Colorectal Adenocarcinoma Cells

Searching for a link between inflammation and colon cancer, we have found that the inflammatory mediator leukotriene D4 (LTD4), via its receptor CysLT1, induces cyclooxygenase-2 expression, survival, and proliferation in intestinal epithelial cells. In conjunction with our previous observation that CysLT1 receptor expression is increased in colorectal adenocarcinomas, we here found an increased nuclear localization of the CysLT1 receptor in colorectal adenocarcinomas. This novel discovery of CysLT1 receptors in the nucleus was further analyzed. It was found to be located in the outer nuclear membrane in colon cancer cells and in the nontransformed epithelial cell line Int 407 cells by Western blot and electron microscopy. Cancer cells displayed higher amounts of the nuclear CysLT1 receptor, but prolonged LTD4 exposure induced its nuclear translocation in nontransformed cells. Truncation of a nuclear localization sequence abrogated this translocation as well as the LTD4-induced proliferative response. In accordance, nuclear CysLT1 receptors exhibited proliferative extracellular signal-regulated kinase 1/2 signaling. The significance of these experimental findings is supported by the observed correlation between the proliferative marker Ki-67 and nuclear CysLT1 receptor localization in colorectal adenocarcinomas. The present findings indicate that LTD4 cannot only be synthesized but also signal proliferation through nuclear CysLT1 receptors, stressing the importance of leukotrienes in inflammation-induced colon carcinogenesis.

Analysis of UV-induced damage and repair in young and senescent human dermal fibroblasts using the comet assay

A major cause of ageing is thought to be the accumulation of damage to macromolecules. Accumulation to DNA damage in cells therefore presupposes that aged cells are unable to repair this damage. We have used the in vitro model of cellular ageing to test the idea that senescent cells are deficient in some aspect of DNA repair. Using the alkaline single cell gel electrophoresis assay (comet assay), we have determined the responses of young and senescent human dermal fibroblasts to DNA damage caused by exposure to UVC light. At low doses of UVC, senescent cells generate smaller comets than young cells whilst at medium doses the situation is reversed. At high doses, young and senescent cells respond similarly to one another. Time course experiments revealing repair of DNA damage show that senescent cells generate larger comets than young cells at early stages of repair suggesting that either senescent cells bear more damage per genome than do young cells or that senescent cells are more efficient at excising bulky adducts from DNA. Cells maintained in low levels of serum irrespective of age are less able to repair DNA damage compared with cells maintained in high levels of serum, and furthermore young and senescent cells maintained in high levels of serum are equally able to repair DNA damage. Our data, therefore, reveal both age-dependent and age-independent responses to UV-induced DNA damage. Use of the comet assay highlights the heterogeneity of cellular responses to genotoxic stress.

Co-localization of PARP-1 and lamin B in the nuclear architecture: A halo-fluorescence- and confocal-microscopy study

A functional interaction between poly(ADP-ribose) polymerase-1 (PARP-1) and lamin B has recently been proposed by nuclear fractionation, crosslinking, and immunoprecipitation experiments. Here we use fluorescence microscopy to verify and extend these findings. We analyze nuclear halo preparations by fluorescence in situ immuno staining (FISIS), which shares attributes with traditional nuclear fractionation techniques, and by confocal laser scanning microscopy (CLSM). The results agree in that a major part of the enzyme co-localizes with lamin B under physiological conditions, where PARP-1 only has basal activity. After DNA damage and the associated activation of PARP-1, and during the subsequent entry into apoptosis, dramatic changes occur: a gradual release of the enzyme from the lamina, accompanied by its accumulation in nucleoli. Our observations are in line with biochemical evidence for lamin B-PARP-1 interactions under physiological conditions and suggest ways by which these interactions are modified to support PARP-functions in damage and its fate in apoptosis.

Laminopathies

Nuclear lamins form a fibrous nucleoskeletal network of intermediate-sized filaments that underlies the inner nuclear membrane. It associates with this membrane through interactions with specific integral nuclear membrane proteins, while within this flattened lamin lattice the nuclear pore complexes are embedded. Next to this peripheral network, the lamins can form intranuclear structures. The lamins are the evolutionary progenitors of the cytoplasmic intermediate filament proteins and have profound influences on nuclear structure and function. These influences require that lamins have dynamic properties and dual identities as structural building blocks on the one hand, and transcription regulators on the other. Which of these identities underlies the laminopathies, a myriad of genetic diseases caused by mutations in lamins or lamin-associated proteins, is a topic of intense debate.

Immunolocalization of detergent-susceptible nucleoplasmic lamin A/C foci by a novel monoclonal antibody

The A-type lamins are localized in the interior of the nucleus as well as on the nuclear periphery. In this study, we have characterized a monoclonal antibody LA-2F9 produced against recombinant rat lamin A which stains a subpopulation of various cell types in a pattern of small nucleoplasmic foci that are unusually susceptible to mild detergent/salt extraction. The specific reactivity of mAb LA-2F9 towards lamins was confirmed by immunoblotting of HeLa and C3H10T(1/2) whole cell lysates and nuclear lysates. The epitope for LA-2F9 was narrowed down to amino acid residues 268-278 (SAKLDNARQSA). To check whether the appearance of lamin foci was cell-cycle-dependent, C3H10T(1/2) cells were serum-starved and then refed to trigger cells to enter the G(1) phase of the cell-cycle. The intensity of staining increased 3.5-fold within 6 h of refeeding, when the maximum number of cells were labeled with LA-2F9. We also checked whether the LA-2F9 foci colocalized with nuclear proteins known to be distributed in small foci such as hnRNPs, snRNPs, SC-35, and p80 coilin, but did not find evidence of colocalization. Our studies suggest that LA-2F9 has a novel and specific reactivity towards detergent-susceptible lower order lamin structures that are likely to be assembly intermediates.

Temporal and spatial variations in structural protein expression during the progression from stunned to hibernating myocardium

BACKGROUND

Dysfunctional and normally perfused remote regions show equal myolysis and glycogen accumulation in pig hibernating myocardium. We tested the hypothesis that these arose secondary to elevations in preload rather than ischemia.

METHODS AND RESULTS

Expression of structural protein (desmin, desmoplakin, titin, cardiotin, alpha-smooth muscle actin, lamin-A/C, and lamin-B2) in viable dysfunctional myocardium was analyzed by immunohistochemistry. We performed blinded analysis of paired dysfunctional left anterior descending coronary artery and normal remote subendocardial samples from stunned (24 hours; n=6), and hibernating (2 weeks; n=6) myocardium versus sham controls pigs (n=7). Within 24 hours, cardiac myocytes globally reexpressed alpha-smooth muscle actin. In stunned myocardium, cardiotin was globally reduced, whereas reductions in desmin were restricted to the dysfunctional region. Alterations progressed with the transition to hibernating myocardium, in which desmin, cardiotin, and titin were globally reduced. A qualitatively similar reorganization of cytoskeletal proteins occurred 3 hours after transient elevation of left ventricular end-diastolic pressure to 33+/-3 mm Hg.

CONCLUSIONS

Qualitative cardiomyocyte remodeling similar to that in humans with chronic hibernation occurs rapidly after a critical coronary stenosis is applied, as well as after transient elevations in left ventricular end-diastolic pressure in the absence of ischemia. Thus, reorganization of cytoskeletal proteins in patients with viable dysfunctional myocardium appears to reflect chronic and/or cyclical elevations in preload associated with episodes of spontaneous regional ischemia.

Formation of nuclear splicing factor compartments is independent of lamins A/C

Nuclear lamins are major architectural elements of the mammalian cell nucleus, and they have been implicated in the functional organization of the nuclear interior, possibly by providing structural support for nuclear compartments. Colocalization studies have suggested a structural role for lamins in the formation and maintenance of pre-mRNA splicing factor compartments. Here, we have directly tested this hypothesis by analysis of embryonic fibroblasts from knock-out mice lacking A- and C-type lamins. We show that the morphology and cellular properties of splicing factor compartments are independent of A- and C-type lamins. Genetic loss of lamins A/C has no effect on the cellular distribution of several pre-mRNA splicing factors and does not affect the compartment morphology as examined by light and electron microscopy. The association of splicing factors with the nuclear matrix fraction persists in the absence of lamins A/C. Live cell microscopy demonstrates that the intranuclear positional stability of splicing factor compartments is maintained and that the exchange dynamics of SF2/ASF between the compartments and the nucleoplasm is not affected by loss of lamin A/C. Our results demonstrate that formation and maintenance of intranuclear splicing factor compartments is independent of lamins A/C, and they argue against an essential structural role of lamins A/C in splicing factor compartment morphology.

A Phosphorylation Cluster in the Chromatin-binding Region Regulates Chromosome Association of LAP2α

LAP2alpha is a LEM family protein associated with nucleoplasmic A-type lamins and chromatin in interphase. Like lamins and other lamina proteins LAP2alpha is cytoplasmic in metaphase, but it associates with chromosomes prior to nuclear envelope formation in late anaphase to telophase. In vitro phosphorylation analysis and mass spectrometry identified a cluster of at least three mitotic cyclin-dependent kinase 1 phosphorylation sites in the C-terminal chromatin-binding region of LAP2alpha as well as four additional potential sites in the cluster, some of which were targeted alternatively in LAP2alpha mutated at the major sites. LAP2alpha mutants containing serine --> alanine mutations at all seven sites revealed a clear phenotype. Mutated LAP2alpha remained associated with chromosomes throughout mitosis, but the dissociation of lamins into the cytoplasm and nuclear envelope disassembly were not affected. These data demonstrate the in vivo significance of mitotic phosphorylation for the dynamic behavior of LAP2alpha in the cell cycle and show that, unlike the interaction with lamins, the chromatin association of LAP2alpha is regulated by multiple mitosis-specific phosphorylation at sites clustered within a defined region in the C terminus of the protein.

A-type lamins regulate retinoblastoma protein function by promoting subnuclear localization and preventing proteasomal degradation

The retinoblastoma protein (pRB) is a critical regulator of cell proliferation and differentiation and an important tumor suppressor. In the G(1) phase of the cell cycle, pRB localizes to perinucleolar sites associated with lamin A/C intranuclear foci. Here, we examine pRB function in cells lacking lamin A/C, finding that pRB levels are dramatically decreased and that the remaining pRB is mislocalized. We demonstrate that A-type lamins protect pRB from proteasomal degradation. Both pRB levels and localization are restored upon reintroduction of lamin A. Lmna(-/-) cells resemble Rb(-/-) cells, exhibiting altered cell-cycle properties and reduced capacity to undergo cell-cycle arrest in response to DNA damage. These findings establish a functional link between a core nuclear structural component and an important cell-cycle regulator. They further raise the possibility that altered pRB function may be a contributing factor in dystrophic syndromes arising from LMNA mutation.

Aging of Hutchinson–Gilford progeria syndrome fibroblasts is characterised by hyperproliferation and increased apoptosis

Hutchinson-Gilford progeria syndrome is a rare genetic disorder that mimics certain aspects of aging prematurely. Recent work has revealed that mutations in the lamin A gene are a cause of the disease. We show here that cellular aging of Hutchinson-Gilford progeria syndrome fibroblasts is characterised by a period of hyperproliferation and terminates with a large increase in the rate of apoptosis. The occurrence of cells with abnormal nuclear morphology reported by others is shown to be a result of cell division since the fraction of these abnormalities increases with cellular age. Similarly, the proportion of cells with an abnormal or absent A-type lamina increases with age. These data provide clues as to the cellular basis for premature aging in HGPS and support the view that cellular senescence and tissue homeostasis are important factors in the normal aging process.

The nuclear lamina and its functions in the nucleus

The nuclear lamina is a structure near the inner nuclear membrane and the peripheral chromatin. It is composed of lamins, which are also present in the nuclear interior, and lamin-associated proteins. The increasing number of proteins that interact with lamins and the compound interactions between these proteins and chromatin-associated proteins make the nuclear lamina a highly complex but also a very exciting structure. The nuclear lamina is an essential component of metazoan cells. It is involved in most nuclear activities including DNA replication, RNA transcription, nuclear and chromatin organization, cell cycle regulation, cell development and differentiation, nuclear migration, and apoptosis. Specific mutations in nuclear lamina genes cause a wide range of heritable human diseases. These diseases include Emery-Dreifuss muscular dystrophy, limb girdle muscular dystrophy, dilated cardiomyopathy (DCM) with conduction system disease, familial partial lipodystrophy (FPLD), autosomal recessive axonal neuropathy (Charcot-Marie-Tooth disorder type 2, CMT2), mandibuloacral dysplasia (MAD), Hutchison Gilford Progeria syndrome (HGS), Greenberg Skeletal Dysplasia, and Pelger-Huet anomaly (PHA). Genetic analyses in Caenorhabditis elegans, Drosophila, and mice show new insights into the functions of the nuclear lamina, and recent structural analyses have begun to unravel the molecular structure and assembly of lamins and their associated proteins.

Failure of lamin A/C to functionally assemble in R482L mutated familial partial lipodystrophy fibroblasts: altered intermolecular interaction with emerin and implications for gene transcription

Familial partial lipodystrophy is an autosomal dominant disease caused by mutations of the LMNA gene encoding alternatively spliced lamins A and C. Abnormal distribution of body fat and insulin resistance characterize the clinical phenotype. In this study, we analyzed primary fibroblast cultures from a patient carrying an R482L lamin A/C mutation by a morphological and biochemical approach. Abnormalities were observed consisting of nuclear lamin A/C aggregates mostly localized close to the nuclear lamina. These aggregates were not bound to either DNA-containing structures or RNA splicing intranuclear compartments. In addition, emerin did not colocalize with nuclear lamin A/C aggregates. Interestingly, emerin failed to interact with lamin A in R482L mutated fibroblasts in vivo, while the interaction with lamin C was preserved in vitro, as determined by coimmunoprecipitation experiments. The presence of lamin A/C nuclear aggregates was restricted to actively transcribing cells, and it was increased in insulin-treated fibroblasts. In fibroblasts carrying lamin A/C nuclear aggregates, a reduced incorporation of bromouridine was observed, demonstrating that mutated lamin A/C in FPLD cells interferes with RNA transcription.

LMNA mutations in atypical Werner's syndrome

BACKGROUND

Werner's syndrome is a progeroid syndrome caused by mutations at the WRN helicase locus. Some features of this disorder are also present in laminopathies caused by mutant LMNA encoding nuclear lamin A/C. Because of this similarity, we sequenced LMNA in individuals with atypical Werner's syndrome (wild-type WRN).

METHODS

Of 129 index patients referred to our international registry for molecular diagnosis of Werner's syndrome, 26 (20%) had wildtype WRN coding regions and were categorised as having atypical Werner's syndrome on the basis of molecular criteria. We sequenced all exons of LMNA in these individuals. Mutations were confirmed at the mRNA level by RT-PCR sequencing. In one patient in whom an LMNA mutation was detected and fibroblasts were available, we established nuclear morphology and subnuclear localisation.

FINDINGS

In four (15%) of 26 patients with atypical Werner's syndrome, we noted heterozygosity for novel missense mutations in LMNA, specifically A57P, R133L (in two people), and L140R. The mutations altered relatively conserved residues within lamin A/C. Fibroblasts from the patient with the L140R mutation had a substantially enhanced proportion of nuclei with altered morphology and mislocalised lamins. Individuals with atypical Werner's syndrome with mutations in LMNA had a more severe phenotype than did those with the disorder due to mutant WRN.

INTERPRETATION

Our findings indicate that Werner's syndrome is molecularly heterogeneous, and a subset of the disorder can be judged a laminopathy.

Reversible changes in the nuclear lamina induced by hyperthermia

The nuclear matrix (NM) has been identified as a potential target for heat-induced cell killing. Previous studies have shown that heat-shock may significantly modulate lamin B content. Since changes in NM structure have often been accompanied by changes in protein composition, we investigated whether hyperthermia induced changes in nuclear lamina (NL) structure in non-tolerant and thermotolerant cells, and the implications of these changes on cell survival. Using indirect immunofluorescence techniques and confocal microscopy, we found that heating cells at 42 or 45.5 degrees C caused invaginations and other distortions of the peripheral NL. While hyperthermia did not alter the number or structure of internal lamin B foci, heat-induced alterations to the peripheral NL were dose-dependent. Interestingly, NL structure recovered with time after heating in cells that were destined to live or die. Thermotolerant cells heated at 45.5 degrees C showed similar initial changes in the NL compared to non-tolerant cells, but recovery occurred much faster. Taken together, these results suggest that the amount of initial damage to the peripheral NL is not correlated with heat-induced cell killing. However, the possibility that an increased rate of recovery might confer a survival advantage cannot be discounted.

Lamin expression in normal human skin, actinic keratosis, squamous cell carcinoma and basal cell carcinoma

BACKGROUND

Aberrant expression patterns of nuclear lamins have been described in various types of cancer depending on the subtype of cancer, its aggressiveness, proliferative capacity and degree of differentiation. In general, the expression of A-type lamins (lamins A and C) has been correlated with a non-proliferating, differentiated state of cells and tissues.

OBJECTIVES

To establish and compare the expression patterns of lamins in normal human skin, actinic keratosis (AK), squamous cell carcinoma (SCC) and basal cell carcinoma (BCC).

METHODS

Expression patterns of the individual lamin subtypes were studied immunohistochemically. The proliferation capacity of the tumour cells was detected using a specific antibody to Ki-67, and was related to the A-type lamin expression patterns.

RESULTS

In normal skin, lamin A was expressed in the suprabasal cell compartment of the epidermis, whereas the basal cells were mostly unstained. BCCs and SCCs stained positive in most cells, while the epidermis overlying BCC and SCC and the epidermis in AK stained homogeneously and strongly in the basal cells in addition to the suprabasal cells. Lamin C was expressed in some basal cells of normal epidermis while the suprabasal cells stained strongly positive. Both BCCs and SCCs stained strongly positive for lamin C, with the difference that in BCC the staining was predominantly present in nucleolar structures with occasional staining of the nuclear envelope. The epidermis overlying SCC showed strong positivity in the lamina of virtually all cells. The expression of lamin C in the basal cells of AK resembled the expression pattern seen in the epidermis overlying BCC, i.e. a nucleolar staining next to nuclear envelope staining. Lamin B1 and B2 were found in virtually all cells in normal epidermis, AK, BCC, SCC and the epidermis overlying cancer. The percentage of Ki-67-expressing cells was highest in BCC (45%), and gradually decreased via epidermis overlying BCC, AK, SCC, and epidermis overlying SCC, to normal skin (11%). Simultaneous expression of A-type lamins and Ki-67 occurred in approximately 50% of the proliferating (Ki-67 positive) cells in BCC and SCC.

CONCLUSIONS

Significant changes occur in the expression patterns of A-type lamins in both premalignant and malignant lesions of the skin. The profound overlap of lamin A and Ki-67 staining patterns indicates that the proliferating tumour cells may obtain a certain degree of differentiation. Finally, lamin A expression in the basal cell layer of the apparently normal epidermis overlying BCC may suggest its involvement in the primary process.

Lamin A/C speckles mediate spatial organization of splicing factor compartments and RNA polymerase II transcription

The A-type lamins have been observed to colocalize with RNA splicing factors in speckles within the nucleus, in addition to their typical distribution at the nuclear periphery. To understand the functions of lamin speckles, the effects of transcriptional inhibitors known to modify RNA splicing factor compartments (SFCs) were examined. Treatment of HeLa cells with alpha-amanitin or 5,6-dichlorobenzimidazole riboside (DRB) inhibited RNA polymerase II (pol II) transcription and led to the enlargement of lamin speckles as well as SFCs. Removal of the reversible inhibitor DRB resulted in the reactivation of transcription and a rapid, synchronous redistribution of lamins and splicing factors to normal-sized speckles, indicating a close association between lamin speckles and SFCs. Conversely, the expression of NH2-terminally modified lamin A or C in HeLa cells brought about a loss of lamin speckles, depletion of SFCs, and down-regulation of pol II transcription without affecting the peripheral lamina. Our results suggest a unique role for lamin speckles in the spatial organization of RNA splicing factors and pol II transcription in the nucleus.

Partial cleavage of A-type lamins concurs with their total disintegration from the nuclear lamina during apoptosis

Although activated caspase 6 is capable of cleaving both A- and B-type lamins during apoptosis, the higher-order structure of the nuclear lamina may cause a differential breakdown of these two types of lamins. In order to obtain a better understanding of the dynamics and the consequences of the rapid, coordinated breakdown of the lamina complex, we applied the green fluorescent protein (GFP) technology in living cells, in which the fate of individual caspase cleavage fragments of A- and B-type lamins was examined. CHO-K1 cells were stably transfected with cDNA constructs encoding N-terminally GFP-labelled hybrids of lamin A, lamin Adelta10, lamin C or lamin B1. The course of the apoptotic process, induced by the kinase inhibitor staurosporine or by the proteasome inhibitor MG132, was monitored by digital imaging microscopy or confocal microscopy. Time-lapse recordings showed that parallel to DNA condensation N-terminally GFP-tagged A-type lamins became diffusely dispersed throughout the nucleoplasm and rapidly translocated to the cytoplasm. In contrast, the majority of GFP-lamin B1 signal remained localised at the nuclear periphery, even after extensive DNA condensation. Comparison of lamin B1-GFP signal with A-type lamin antibody staining in the same apoptotic cells confirmed the temporal differences between A- and B-type lamina dispersal. Immunoblotting revealed only a partial cleavage of A-type lamins and an almost complete cleavage of lamin B1 during apoptosis. In contrast to lamin B1 in normal cells, this cleaved lamin B1, which is apparently still associated with the nuclear membrane, can be completely extracted by methanol or ethanol. Fluorescence loss of intensity after photobleaching experiments showed that in apoptotic cells A-type lamin-GFP molecules diffuse almost freely in both nucleoplasm and cytoplasm, while the lamin B1-GFP fragments remain more stably associated with the nuclear membrane, which is confirmed by co-localisation immunofluorescence studies with a nucleoporin p62 antibody. Our results therefore clearly show a differential behaviour of A- and B-type lamins during apoptosis, suggesting not only distinct differences in the organisation of the lamina filaments, but also that caspase cleavage of only a small fraction of A-type lamins is needed for its complete disintegration.

Lamins: building blocks or regulators of gene expression?

Intermediate filament (IF) proteins are the building blocks of cytoskeletal filaments, the main function of which is to maintain cell shape and integrity. The lamins are thought to be the evolutionary progenitors of IF proteins and they have profound influences on both nuclear structure and function. These influences require the lamins to have dynamic properties and dual identities--as building blocks and transcriptional regulators. Which one of these identities underlies a myriad of genetic diseases is a topic of intense debate.

Distinct functions of the unique C terminus of LAP2alpha in cell proliferation and nuclear assembly

The non-membrane-bound lamina-associated polypeptide 2 isoform, LAP2alpha, forms nucleoskeletal structures with A-type lamins and interacts with chromosomes in a cell cycle-dependent manner. LAP2alpha contains a LEM (LAP2, emerin, and MAN1) domain in the constant N terminus that binds to chromosomal barrier-to-autointegration factor, and a C-terminal unique region that is essential for chromosome binding. Here we show that C-terminal LAP2alpha fragment efficiently bound to mitotic chromosomes and inhibited assembly of endogenous LAP2alpha, nuclear membranes, and lamins A/C in in vitro nuclear assembly assays. Full-length recombinant LAP2alpha, which bound to chromosomes, and N-terminal fragment, which did not bind, had no effect on assembly. This suggested an essential role for the LAP2alpha C terminus in chromosome association and for the N-terminal LEM domain in subsequent assembly stages. In vivo analysis upon transient expression of GFP-tagged LAP2alpha fragments confirmed that, unlike the N-terminal fragment, the C-terminal fragment was able to bind to chromosomes during mitosis, if expressed weakly. At higher expression levels, C-terminal LAP2alpha fragment and full-length protein led to cell cycle arrest in interphase and apoptosis, as shown by fluorescence-activated cell sorter analysis, time lapse microscopy, and BrdUrd incorporation assays. These data indicated distinct functions of LAP2alpha in cell cycle progression during interphase and in nuclear reassembly during mitosis.

Alteration of nuclear lamin organization inhibits RNA polymerase II-dependent transcription

Regulation of gene activity is mediated by alterations in chromatin organization. In addition, chromatin organization may be governed in part by interactions with structural components of the nucleus. The nuclear lamins comprise the lamina and a variety of nucleoplasmic assemblies that together are major structural components of the nucleus. Furthermore, lamins and lamin-associated proteins have been reported to bind chromatin. These observations suggest that the nuclear lamins may be involved in the regulation of gene activity. In this report, we test this possibility by disrupting the normal organization of nuclear lamins with a dominant negative lamin mutant lacking the NH2-terminal domain. We find that this disruption inhibits RNA polymerase II activity in both mammalian cells and transcriptionally active embryonic nuclei from Xenopus laevis. The inhibition appears to be specific for polymerase II as disruption of lamin organization does not detectably inhibit RNA polymerases I and III. Furthermore, immunofluorescence observations indicate that this selective inhibition of polymerase II-dependent transcription involves the TATA binding protein, a component of the basal transcription factor TFIID.

Somatic lamins in germinal vesicles of goldfish (Carassius auratus) vitellogenic oocytes

In fish and amphibians, B-type lamins are divided into somatic (B1, B2) and oocyte-type (B3) lamins. In this study, we purified nuclear lamins from rainbow trout erythrocytes, raised an anti-lamin monoclonal antibody (L-200) that recognizes goldfish somatic-lamins, and isolated cDNAs encoding goldfish B-type lamins (B1 and B2) from a goldfish cell culture cDNA library. Goldfish B-type lamins are structurally similar to lamins found in other vertebrates with minor amino acid substitutions in the conserved region. Western blot analysis showed that goldfish oocytes contained mainly GV-lamin B3 as well as some somatic lamins. Laser-confocal microscope observations revealed that lamin B3 was present only in GV nuclear lamina, whereas somatic lamins were present in dense fibrillar structures throughout nuclear gels of isolated GVs. Similar nuclear filamentous structures were also observed in GVs of paraffin embedded oocytes. Epitope mapping indicated that L-200 recognized a conserved region containing a short stretch of the alpha-helix coiled-coil rod domain (Y(E/Q)(Q/E)LL). A similar motif is also present in other cytoplasmic intermediate filaments (i.e., vimentin, desmin, peripherin and GFAP). Taken together, these findings suggest that lamins or lamin-related intermediate filaments are an important component of the interior architecture of goldfish vitellogenic oocyte nuclei (GVs).

Distinct changes in intranuclear lamin A/C organization during myoblast differentiation

Intranuclear lamin foci or speckles have been observed in various cell types. In order to explore the possibility of changes in internal lamin organization during muscle differentiation, we have examined the appearance of A-type lamin speckles that associate with RNA splicing factor speckles in C2C12 myoblasts and myotubes. Lamin speckles were observed in dividing myoblasts but disappeared early during the course of differentiation in postmitotic myocytes, and were absent in myotubes and muscle fibers. However, no changes were seen in the typical peripheral organization of lamins A/C or B1 or in RNA splicing factor speckles. Lamin speckles were also absent in quiescent myoblasts but reappeared as cells were reactivated to enter the cell cycle. These changes were not observed in other quiescent cell types. Immunoblot analysis indicated that the abundance and migration of lamins A and C was not altered in differentiated myoblasts. When myotube or quiescent myoblast nuclei were extracted with nucleases and detergent, a uniformly stained internal lamina was revealed, indicating that lamins A/C were antigenically masked in these cells, probably owing to structural reorganization of the lamina during differentiation or quiescence. Our results suggest that muscle cell differentiation is accompanied by regulated rearrangements in the organization of the A-type lamins.

Novel properties of the cyclin encoded by Human Herpesvirus 8 that facilitate exit from quiescence

Viral DNA replication is generally dependent upon circumventing host cell cycle control to force S phase entry in an otherwise quiescent cell. Here we describe novel attributes of the cyclin encoded by Human Herpesvirus 8 (K cyclin) that enable it to subvert the quiescent state. K cyclin is most similar to the mammalian D-type cyclins in primary sequence but displays properties more akin to those of cyclin E. K cyclin (like cyclin E) can autonomously couple with its cognate cdk subunit and localize to the nucleus. D-type cyclins require mitogen stimulated accessory factors (such as p21(Cip1) and p27(Kip1)) to facilitate both of these processes. A striking difference between K cyclin and mammalian cyclins is that K cyclin binding to cdk6 can substantially activate the catalytic activity of the complex without the requirement for cyclin H/cdk7 phosphorylation of the cdk T-loop; this phosphorylation is obligatory for endogenous cyclin/cdk activity. However, K cyclin/cdk6 complexes are not totally immune from cell cycle control since CAK phosphorylation is necessary for complete activation. Thus, CAK phosphorylated K cyclin/cdk6 targets multiple sites in the retinoblastoma protein (pRb) whereas the unphosphorylated complex targets a single site. The restricted substrate specificity of the non-CAK phosphorylated K cyclin/cdk6 complex is insufficient to enable K cyclin-mediated S phase entry. Thus, the viral K cyclin is reliant upon endogenous CAK activity to subvert the quiescent state.

Lamins in disease: why do ubiquitously expressed nuclear envelope proteins give rise to tissue-specific disease phenotypes?

The nuclear lamina is a filamentous structure composed of lamins that supports the inner nuclear membrane. Several integral membrane proteins including emerin, LBR, LAP1 and LAP2 bind to nuclear lamins in vitro and can influence lamin function and dynamics in vivo. Results from various studies suggest that lamins function in DNA replication and nuclear envelope assembly and determine the size and shape of the nuclear envelope. In addition, lamins also bind chromatin and certain DNA sequences, and might influence chromosome position. Recent evidence has revealed that mutations in A-type lamins give rise to a range of rare, but dominant, genetic disorders, including Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy with conduction-system disease and Dunnigan-type familial partial lipodystrophy. An examination of how lamins A/C, emerin and other integral membrane proteins interact at the INM provides the basis for a novel model for how mutations that promote disease phenotypes are likely to influence these interactions and therefore cause cellular pathology through a combination of weakness of the lamina or altered gene expression.

Nuclear lamins A and B1: different pathways of assembly during nuclear envelope formation in living cells

At the end of mitosis, the nuclear lamins assemble to form the nuclear lamina during nuclear envelope formation in daughter cells. We have fused A- and B-type nuclear lamins to the green fluorescent protein to study this process in living cells. The results reveal that the A- and B-type lamins exhibit different pathways of assembly. In the early stages of mitosis, both lamins are distributed throughout the cytoplasm in a diffusible (nonpolymerized) state, as demonstrated by fluorescence recovery after photobleaching (FRAP). During the anaphase-telophase transition, lamin B1 begins to become concentrated at the surface of the chromosomes. As the chromosomes reach the spindle poles, virtually all of the detectable lamin B1 has accumulated at their surfaces. Subsequently, this lamin rapidly encloses the entire perimeter of the region containing decondensing chromosomes in each daughter cell. By this time, lamin B1 has assembled into a relatively stable polymer, as indicated by FRAP analyses and insolubility in detergent/high ionic strength solutions. In contrast, the association of lamin A with the nucleus begins only after the major components of the nuclear envelope including pore complexes are assembled in daughter cells. Initially, lamin A is found in an unpolymerized state throughout the nucleoplasm of daughter cell nuclei in early G1 and only gradually becomes incorporated into the peripheral lamina during the first few hours of this stage of the cell cycle. In later stages of G1, FRAP analyses suggest that both green fluorescent protein lamins A and B1 form higher order polymers throughout interphase nuclei.

Head and/or CaaX domain deletions of lamin proteins disrupt preformed lamin A and C but not lamin B structure in mammalian cells

The nuclear lamina is an important determinant of nuclear architecture. Mutations in A-type but not B-type lamins cause a range of human genetic disorders, including muscular dystrophy. Dominant mutations in nuclear lamin proteins have been shown to disrupt a preformed lamina structure in Xenopus egg extracts. Here, a series of deletion mutations in lamins A and B1 were evaluated for their ability to disrupt lamina structure in Chinese hamster ovary cells. Deletions of either the lamin A "head" domain or the C-terminal CaaX domain formed intranuclear aggregates and resulted in the disruption of endogenous lamins A/C but not lamins B1/B2. By contrast, "head-less" lamin B1 localized to the nuclear rim with no detectable effect on endogenous lamins, whereas lamin B1 CaaX domain deletions formed intranuclear aggregates, disrupting endogenous lamins A/C but not lamins B1/B2. Filter binding assays revealed that a head/CaaX domain lamin B1 mutant interacted much more strongly with lamins A/C than with lamins B1/B2. Regulated induction of this mutant in stable cell lines resulted in the rapid elimination of all detectable lamin A protein, whereas lamin C was trapped in a soluble form within the intranuclear aggregates. In contrast to results in Xenopus egg extracts, dominant negative lamin B1 (but not lamin A) mutants trapped replication proteins involved in both the initiation and elongation phases of replication but did not effect cellular growth rates or the assembly of active replication centers. We conclude that elimination of the CaaX domain in lamin B1 and elimination of either the CaaX or head domain in lamin A constitute dominant mutations that can disrupt A-type but not B-type lamins, highlighting important differences in the way that A- and B-type lamins are integrated into the lamina.

Nuclear organization of DNA replication in primary mammalian cells

Using methods that conserve nuclear architecture, we have reanalyzed the spatial organization of the initiation of mammalian DNA synthesis. Contrary to the commonly held view that replication begins at hundreds of dispersed nuclear sites, primary fibroblasts initiate synthesis in a limited number of foci that contain replication proteins, surround the nucleolus, and overlap with previously identified internal lamin A/C structures. These foci are established in early G(1)-phase and also contain members of the retinoblastoma protein family. Later, in S-phase, DNA replication sites distribute to regions located throughout the nucleus. As this progression occurs, association with the lamin structure and pRB family members is lost. A similar temporal progression is found in all the primary cells we have examined but not in most established cell lines, indicating that the immortalization process modifies spatial control of DNA replication. These findings indicate that in normal mammalian cells, the onset of DNA synthesis is coordinately regulated at a small number of previously unrecognized perinucleolar sites that are selected in early G(1)-phase.

Essential roles for Caenorhabditis elegans lamin gene in nuclear organization, cell cycle progression, and spatial organization of nuclear pore complexes

Caenorhabditis elegans has a single lamin gene, designated lmn-1 (previously termed CeLam-1). Antibodies raised against the lmn-1 product (Ce-lamin) detected a 64-kDa nuclear envelope protein. Ce-lamin was detected in the nuclear periphery of all cells except sperm and was found in the nuclear interior in embryonic cells and in a fraction of adult cells. Reductions in the amount of Ce-lamin protein produce embryonic lethality. Although the majority of affected embryos survive to produce several hundred nuclei, defects can be detected as early as the first nuclear divisions. Abnormalities include rapid changes in nuclear morphology during interphase, loss of chromosomes, unequal separation of chromosomes into daughter nuclei, abnormal condensation of chromatin, an increase in DNA content, and abnormal distribution of nuclear pore complexes (NPCs). Under conditions of incomplete RNA interference, a fraction of embryos escaped embryonic arrest and continue to develop through larval life. These animals exhibit additional phenotypes including sterility and defective segregation of chromosomes in germ cells. Our observations show that lmn-1 is an essential gene in C. elegans, and that the nuclear lamins are involved in chromatin organization, cell cycle progression, chromosome segregation, and correct spacing of NPCs.

Lamina-associated polypeptide 2alpha binds intranuclear A-type lamins

The nucleoskeletal protein lamina-associated polypeptide 2(α) (LAP2*) contains a large, unique C terminus and differs significantly from its alternatively spliced, mostly membrane-integrated isoforms, such as LAP2beta. Unlike lamin B-binding LAP2beta, LAP2alpha was found by confocal immunofluorescence microscopy to colocalize preferentially with A-type lamins in the newly formed nuclei assembled after mitosis. While only a subfraction of lamins A and C (lamin A/C) was associated with the predominantly nuclear LAP2alpha in telophase, the majority of lamin A/C colocalized with LAP2alpha in G(1)-phase nuclei. Furthermore, selective disruption of A-type lamin structures by overexpression of lamin mutants in HeLa cells caused a redistribution of LAP2alpha. Coimmunoprecipitation experiments revealed that a fraction of lamin A/C formed a stable, SDS-resistant complex with LAP2alpha in interphase cells and in postmetaphase cell extracts. Blot overlay binding studies revealed a direct binding of LAP2alpha to exclusively A-type lamins and located the interaction domains to the C-terminal 78 amino acids of LAP2alpha and to residues 319–566 in lamin A/C, which include the C terminus of the rod and the entire tail common to lamin A/C. These findings suggest that LAP2alpha and A-type lamins cooperate in the organization of internal nuclear structures.